The present invention relates to protective coatings for components exposed to high temperatures, such as components of a gas turbine engine. More particularly, this invention is directed to a method for removing and refurbishing a thermal barrier coating system that includes an inner metallic bond coat and an outer thermal-insulating ceramic layer.
The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature alloys have been achieved through the formulation of iron, nickel and cobalt-base superalloys, though components formed from such alloys often cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the turbine, combustor or augmentor. A common solution is to protect the surfaces of such components with an environmental coating system, such as an aluminide coating or a thermal barrier coating system (TBC). The latter includes an environmentally-resistant bond coat and a layer of thermal insulating ceramic applied over the bond coat. Bond coats are typically formed from an oxidation-resistant alloy such as MCrAlY where M is iron, cobalt and/or nickel or from a diffusion aluminide or platinum aluminide that forms an oxidation-resistant intermetallic. During high temperature excursions, these bond coats form an oxide layer or scale that chemically bonds the ceramic layer to the bond coat. Zirconia (ZrO2) that is partially or fully stabilized by yttria (Y2O3), magnesia (MgO) or other oxides has been widely employed as the material for the ceramic layer. The ceramic layer is typically deposited by air plasma spraying (APS), low pressure plasma spraying (LPPS), or a physical vapor deposition (PVD) technique, such as electron beam physical vapor deposition (EBPVD) which yields a strain-tolerant columnar grain structure.
Though significant advances have been made with coating materials and processes for producing both the environmnentally-resistant bond coat and the thermal insulating ceramic layer, there is the inevitable requirement to remove and replace the ceramic layer under certain circumstances. For example, removal may be necessitated by erosion or impact damage to the ceramic layer during engine operation, or by a requirement to repair certain features such as the tip length of a turbine blade. Removal of the ceramic layer may also be necessitated during component manufacturing to address such problems as defects in the coating, handling damage and the need to repeat noncoating-related manufacturing operations which require removal of the ceramic, e.g., electrical-discharge machining (EDM) operations.
The current state-of-the-art repair methods often result in removal of the entire TBC system, i.e., both the ceramic layer and bond coat, after which the bond coat and ceramic layer must be redeposited. Prior art abrasive techniques for removing thermal barrier coatings have generally involved grit blasting, vapor honing and glass bead peening, each of which is a slow, labor-intensive process that erodes the ceramic layer and bond coat, as well as the substrate surface beneath the coating. With repetitive use, these removal processes eventually destroy the component by erosion. Damage is particularly likely when treating an air-cooled turbine blade, whose surface includes cooling holes from which cooling air is discharged in order to cool the external surfaces of the blade.
Consequently, significant effort has been directed to developing nonabrasive processes for removing ceramic coatings. One such method disclosed in U.S. Pat. No. 4,889,589 involves the use of a fluoride-containing gas at elevated temperatures. This process removes both ceramic coatings and their aluminide bond coats. Yet another nonabrasive process involves the use of a high pressure waterjet, as reported in U.S. Pat. No. 5,167,721. While this waterjet technique is described as not removing the bond coat, in practice the waterjet can inflict significant damage to bond coats and particularly diffusion aluminide bond coats, which are brittle beneath about 1200xc2x0 F. (about 650xc2x0 C.). Damage generally occurs by the fracturing of brittle phases in the bond coat, such as PtAl2 phases of a platinum-aluminide bond coat, and/or the additive layer, which is the outermost bond coat layer containing an environmentally-resistant intermetallic phase MAl, where M is iron, nickel or cobalt, depending on the substrate material. Similar to grit blasting techniques, bond coat damage from the waterjet process is particularly likely when treating an air-cooled turbine blade. Damage is particularly acute around the cooling holes of these blades because ceramic within the holes is anchored by compressive stresses that develop when the newly coated component cools from typical coating temperatures, e.g., above about 1800xc2x0 F. (about 980xc2x00 C.) for ceramic deposited by PVD techniques. Consequently, to remove the ceramic from a cooling hole, excessive dwell times are required to overcome this strong mechanical bond as well as the chemical bond between the ceramic and oxide layers, resulting in significant damage or removal of the bond coat in and around the cooling holes.
Another nonabrasive process capable of selectively removing a ceramic layer of a TBC system is an autoclaving process in which a turbine blade is subjected to elevated temperatures and pressures in the presence of a caustic compound. This process has been found to sufficiently weaken the chemical bond between the ceramic and bond coat oxide layers to permit removal of the ceramic layer while leaving the bond layer intact. However, as with the previous methods discussed above, this process also is incapable of removing ceramic from the cooling holes of an air-cooled turbine blade. A more recent nonabrasive process for removing a TBC system is disclosed in U.S. patent application Ser. No. 08/362,377 to Reeves et al, assigned to the assignee of this invention. This process entails heating the TBC to about 870xc2x0 C. or more while exposing the coating to a halogen-containing powder, which causes the entire coating to deteriorate to the extent that it separates from the underlying substrate. Both the ceramic layer and the bond coat must be redeposited as a result of the halogen-containing powder attacking each of these layers. However, this process also has been found to be unsuccessful at removing ceramic mechanically held in the cooling holes of a turbine blade.
From the above, it can be seen that the complete removal of the ceramic layer of a TBC system from a turbine engine component is complicated if the component is formed to have cooling holes at its exterior surface. Removal of ceramic from a cooling hole is particularly difficult if the component has a complex cooling scheme in which the cooling holes are carefully configured to discharge a film of air that spreads across the external surfaces of the component. The performance of such a component is directly related to the ability to provide uniform film cooling of the component surfaces. However, air-cooled components that have been coated with an insulating ceramic layer, processed through previous TBC removal techniques, and subsequently recoated with a ceramic layer have unacceptable air film distribution and reduced airflow as a result of the buildup of ceramic in the cooling holes on top of residual ceramic from the initial coating. As discussed above, prior art methods are either unsuccessful at removing the ceramic from a cooling hole or inflict severe damage to the surface area surrounding the cooling hole during the process of removing the ceramic. Subsequent heating of the component prior to, during or after any one of the above-noted removal processes has been found to be insufficient to relax the severe compressive stresses that firmly anchor the ceramic material inside the cooling holes.
Accordingly, what is needed is a process capable of removing a ceramic layer of a TBC system without damaging or removing the underlying bond coat, and further capable of removing ceramic coating material from cooling holes of an air-cooled component.
According to the present invention, there is provided a method of repairing a thermal barrier coating system on a component designed for use in a hostile thermal environment, such as turbine, combustor and augmentor components of a gas turbine engine. This method is particularly suited for completely removing a thermal insulating ceramic layer of a thermal barrier coating system that includes a metallic bond coat, such as a diffusion aluminide or MCrAlY coating, between the surface of the component and the ceramic layer, such that the bond coat remains substantially undamaged from the removal process. In particular, the method of this invention entails a technique by which ceramic material within cooling holes in the component can be removed without damaging the underlying bond coat.
The processing steps of this invention generally include removing the ceramic layer from the surface of the component by subjecting the ceramic layer to a caustic solution at an elevated temperature and pressure, during which the chemical bond between the ceramic layer and the oxide layer on the underlying bond coat is attacked without damaging or removing the bond coat. Thereafter, the method of this invention entails a further step of removing ceramic material from cooling holes present in the component by directing a high-velocity fluid stream into the cooling holes. According to this invention, once the ceramic layer from the surrounding surface area of the component is removed, a sufficiently high-velocity fluid stream has been found to be able to overcome the compressive stresses that otherwise mechanically anchor the ceramic in the cooling hole, yet does not remove or damage the bond coat within the cooling hole.
Using the method of this invention, the ceramic layer is completely removed from the component and any cooling holes, with essentially no degradation of the bond coat. Accordingly, this invention allows deposition of a new ceramic coating on components in production without refurbishment or replacement of the bond coat and without depositing additional ceramic in the cooling holes, which would be detrimental to the performance of the component. If the component has been in service, such that the bond coat has been partially depleted as a result of oxidation, the bond coat can be refurbished by an aluminiding technique, such as pack cementation or vapor phase aluminiding technique. Refurbishment by aluminiding is applicable to both diffusion aluminide and MCrAlY bond coats. Ceramic material can then be deposited on the surface of the refurbished bond coat so as to form a new insulating ceramic layer. The autoclave process of this invention also serves to remove any oxides from internal blade surfaces, allowing for the possibility of rejuvenation of internal coating as well during the aluminizing process.
Importantly, prior art techniques for removing a ceramic layer of a TBC have not enabled the above-noted refurbishment technique for air-cooled components, in that such techniques either cannot remove ceramic from the cooling holes or excessively damage the bond coat in the process. As a result of this invention, the labor, processing and costs required to refurbish a thermal barrier coating system are significantly reduced by avoiding damage and removal of the bond coat. In addition, the service life of a component can also be extended by avoiding replacement of its entire thermal barrier coating system, since removal of a bond coat results in loss of wall thickness because bond coats are inter-diffused with their blade substrates. Another significant advantage is that the process of this invention is less time-consuming and more efficient than if a waterjet were used alone to remove the ceramic layer. Furthermore, because the present invention enables the removal of ceramic from cooling holes, the performance of a component treated according to the process of this invention is promoted by the restored uniform film cooling of the component surfaces.
Other objects and advantages of this invention will be better appreciated from the following detailed description.